1. Field of the Invention
The present invention relates to a control system for regulating a traction motor that generates at least part of the torque of a motor vehicle or a hybrid vehicle.
2. Description of the Related Art
As described in Japanese Patent Publication No. 2003-009573, part or whole torque of a vehicle generated by a traction motor is controlled by regulating its voltage in response to a command signal supplied from a motor ECU (electronic control unit), which provides calculation to generate a current command value and a voltage command value. The current command value is determined based on a torque value supplied from an external ECU and a load value represented by the number of revolutions of the traction motor. The voltage command value is determined by using a feedback control algorithm that reduces the difference between the determined current command value and the actual current of the motor to zero. The voltage command value is supplied to an inverter, where the voltage command value is converted to AC power that drives the traction motor corresponding to the torque command value.
However, if a sudden change occurs in the running load of the vehicle, it can cause excessive slip or lock of the driven wheels, which could result in a loss of vehicle stability.
In a hybrid vehicle, the HV-ECU calculates the necessary torque from accelerator pedal opening, vehicle speed or the number of revolutions (RPM) of the traction motor. The motor ECU receives this calculated torque as a command value and performs a feedback control on the motor current so that it approaches its command value, whereby the traction motor generates a torque corresponding to its torque command value supplied from the HV-ECU. If a sudden change occurs in the running load, the HV-ECU will alter its torque command value in order to compensate for the sudden load change. However, this requires communication between the HV-ECU and the motor ECU and computations, which results in a delay time (typically, 20 milliseconds) and hence a sudden change in the speed of revolutions of the traction motor.
In response to the sudden change in motor speed, the output voltage of the motor and hence the output of the inverter varies rapidly. Eventually, the sudden variation affects the usable lifetime of the battery.
Therefore, a need exists to provide a traction motor control system that assures vehicle stability and avoids sudden change in battery power consumption for extending its lifetime.